1. Field of the Invention
The present invention relates to a honeycomb filter used for trapping or cleaning up particulates contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or various combustion apparatuses and to a method for producing the honeycomb filter.
2. Description of Related Art
A large mount of particulate matter (hereinbelow referred to as “particulate matter”, “particulates”, or “PM”) anchored by soot (graphite) is contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or various combustion apparatuses (hereinbelow appropriately referred to as “internal combustion engine and the like”). Since environmental pollution is caused when the particulates are released without change into the atmosphere, it is general that a filter for trapping particulates is mounted in the exhaust gas passage from the internal combustion engine or the like.
An example of the filter used for such a purpose is a honeycomb filter having a honeycomb structure having a plurality of cells separated by partition walls formed of porous ceramic having a large number of pores and functioning as exhaust gas passages with one side open end portions and the other side open end portions of plural cells being alternately plugged with plugging portions. In such a honeycomb filter, when exhaust gas is sent into the exhaust gas inflow cells (cells not plugged on the exhaust gas inflow side), particulates in the exhaust gas are trapped when exhaust gas passes through the partition walls, and purified gas from which the particulates are removed is discharged from the purified gas outflow cells (cells not plugged on the exhaust gas outflow side).
However, in such a conventional honeycomb filter, there arises a problem of easily causing pressure loss in the partition walls in accordance with a deposition mode of soot or ash. In particular, in order to reduce pressure loss to improve the trapping efficiency, it is effective to impart properties of pores having a small average pore diameter to a honeycomb filter. However, when a layer having such properties is formed on the partition walls of the honeycomb filter, pressure loss of the partition walls is increased when the exhaust gas passes through the partition walls at high flow rates. Therefore, in a conventional honeycomb filter, it is difficult to realize improvement in purification performance and regeneration efficiency simultaneously with planning the reduction of pressure loss.
For the aforementioned problems, there are the following Patent Documents 1 and 2.
The Patent Document 1 discloses a ceramic filter “provided with a fine particle portion having an average pore diameter of 1 to 10 μm and a thickness of at least 10 times the average pore diameter on a surface on one side of a support layer formed of a ceramic filter porous body” for the purpose of providing “a ceramic filter for exhaust gas, the filter having little change of pressure loss with the passage of time after trapping and high trapping efficiency and being excellent on practical side.
The Patent Document 2 discloses a “surface filter for fine particles, the filter having passages selectively clogged and a micro porous membrane imparted to the surfaces of the passages, being formed of a porous honeycomb monolith structure, and being regenerable by reverse flushing” for the purpose of providing “a new filtering apparatus regenerable by a reverse flushing treatment”.
However, the Patent Documents 1 and 2 aim to improve PM-trapping performance by forming a layer having an average pore diameter smaller than that of the partition walls on the partition walls. In such a case, the opening of the cell on the exhaust gas inflow side as an inlet channel becomes small for the thickness of the layer formed therein. Therefore, there arises a problem of remarkable increase in the pressure loss of the partition walls particularly when the exhaust gas passes through the partition walls at high flow rates. On the other hand, reduction of the thickness of the partition wall can be considered for avoiding the problem. However, since thermal capacity is reduced when the thickness of the partition walls is reduced, and inlet temperature (temperature of the exhaust gas inflow side end face in the honeycomb filter) may be varied upon regeneration to become higher than the target temperature. In such a case, since quick combustion of soot may be caused to sharply raise the temperature inside the honeycomb filter, a crack may easily be caused in the honeycomb filter.
As described above, a response to the conventional problems is still insufficient even by the Patent Documents 1 and 2, and a solution in the early stages is desired.